1. Field of the Invention
This invention relates to an improved process for the continuous production of sodium silicate solutions under high pressure and at elevated temperature and to an apparatus for carrying out this process.
2. Statement of the Related Art
Of the alkali metal silicate solutions known as waterglass, sodium silicate solutions, generally called soda waterglass, are the most frequently used for commercial purposes. Soda waterglasses normally have a solids content of approximately 35% by weight and a weight ratio of SiO.sub.2 :Na.sub.2 O of 3.3-3.4:1. Soda waterglasses are industrially produced by fusing quartz sand and soda in suitable furnaces at temperatures in the range 1,400.degree. to 1,500.degree. C. with evolution of CO.sub.2. In another process step, the melt which solidifies on cooling, i.e., "lump glass", is dissolved in water under pressure at elevated temperature and the solution obtained is optionally filtered, depending on the quality requirements. By mixing solutions such as these with sodium hydroxide, it is possible to obtain sodium silicate solutions characterized by a lower weight ratio of SiO.sub.2 to Na.sub.2 O. However, this process is very expensive both in terms of plant and in terms of the energy required.
Another possible method of obtaining soda waterglasses comprises fusing sand hydrothermally with aqueous sodium hydroxide. By this process, it is possible to obtain sodium hydroxide solutions having a SiO.sub.2 :Na.sub.2 O weight ratio of up to 2.8:1. The reaction vessels normally used for hydrothermal syntheses of this type are cylindrical, nickel-lined iron autoclaves which are either mounted for rotation about their horizontal axis or are provided with suitable stirrers to ensure adequate intermixing of the sand/sodium hydroxide suspension. However, this process is attended by disadvantages insofar as, previously, it has not been possible to carry out the process continuously and very long reaction times are necessary to obtain complete conversion of the sand when it is used at moderate, industrially practicable temperatures. In addition, to obtain as large a sand/sodium hydroxide reaction interface as possible, it is important to use a large excess of sand, of which only part dissolves during the reaction and which puts up a high inertial resistance to the stirrers. This resistance as well as the rubbing of the sand against the blades of the stirrers produce serious wear. In addition, separation of the excess sand and purification of the sodium silicate solution formed necessitates subsequent filtration using settling filters and filter aids. This is also expensive and not without problems.
Published German patent application No. 30 02 834 which is commonly assigned with this invention and has some coinventors in common, describes a fluidized bed process for the hydrothermal production of sodium silicate solutions in a static reaction vessel, in which the liquid phase of the reaction mixture is pump-circulated at relatively higher temperatures (150.degree.-250.degree. C.) and pressures through a sand bed introduced into a static reaction vessel (4) and also through a bypass pipe (5) arranged outside the vessel until the concentration of SiO.sub.2 in the liquid phase has reached the desired level. This process is attended by the disadvantage that, in the static reaction vessel which tapers downwards like a funnel and into which the liquid phase of the reaction mixture is pumped in from below, a large part of the sand accumulates in the vicinity of the liquid inlet and on the "funnel" surfaces and only a narrow, upwardly widening reaction zone is formed, in which adequate contact between solid sand and the alkaline reaction solution is achieved as a result of thorough mixing by the pumped-in solution. Accordingly, high temperatures and pressures and comparatively long reaction times are required to reach the desired concentration of SiO.sub.2 in the liquid phase. In the event of a fault in the plant, for example through failure of the liquid feed pump, this narrow reaction zone is also immediately blocked by sedimenting sand and the solid sand/waterglass cake has to be mechanically removed at considerable expense.
Published German patent application No. 31 21 669, which is commonly assigned with this invention and has an inventor in common, describes another continuous process for carrying out heterogeneous reactions involving solid, granular components in a vertical reaction vessel, in which the reaction mixture is fed into the reactor from below against the pressure preferring in the reactor. Uniform dispersion of the various reaction components, particularly the granular reactants, is said to be obtained by mechanical fittings installed in the reaction vessel, for example in the form of perforated plates through which the reaction mixture is forced. Reactors constructed in this way may also be used for hydrothermal reactions for the production of alkali metal silicate solutions, but are attended by the disadvantage that the perforated plates mentioned very quickly become blocked by the reaction component, sand, and are thus unable to contribute towards adequate mixing of the reactants.
Another process for the production of alkali metal silicate solutions in a static reactor is described in published British patent application No. 2,119,779 according to which an aqueous alkali hydroxide solution, preferably a sodium hydroxide solution, is reacted with silicon dioxide in the form of coarse sand in a vertical tubular reactor in which no mechanical agitation takes place. The alkaline solution is fed into the reactor from above, while clear alkali metal silicate solutions can be run off from the bottom of the reactor. The disadvantage of this process is that the alkaline solution only dissolves the sand successively and sand particles decreasing in size cake together, even under the pressure of the flowing solution, to form a solid cake of quartz powder which increases resistance to flow and thus impedes the throughflow of the solution. Another disadvantage is that insoluble impurities remain in the reactor because the sand bed acts as a filter.